Nitrogen plays an important role in various plant functions, including metabolism, resource allocation, growth, and development (Crawford, N. M., “Nitrate: Nutrient and Signal for Plant Growth,” Plant Cell 7:859-868 (1995); Marschner, M., Mineral Nutrition of Higher Plants, 2d ed., Academic Press Ltd.: London (1995); and Stiit et al., “The Molecular Physiological Basis for the Interaction Between Elevated Carbon Dioxide and Nutrients,” Plant Cell Environ. 22:583-622 (1999)). Further, nitrogen is a major component of proteins and nucleic acids, as well as various secondary metabolites found in plants (Marschner, M., Mineral Nutrition of Higher Plants, 2d ed., Academic Press Ltd.: London (1995)). Therefore, nitrogen is one of the most important inorganic nutrients of plants. Inorganic nitrogen is added to many crop plants in the form of nitrogenous fertilizers (see Frink et al., “Nitrogen Fertilizer: Retrospect and Prospect,” Proc. Natl. Acad. Sci. USA 96:1175-1180 (1999)). Nitrogen is principally added to the soil in the form of ammonia (NH4+) and nitrate (NO3−). However, estimates of nitrogen uptake efficiency have shown that between 50 and 70 percent of the applied nitrogen is lost from the plant-soil system (Peoples et al., “Minimizing Gaseous Losses of Nitrogen,” In Nitrogen Fertilizer in the Environment, Bacon, P. E., ed., Marcel Dekker, pp. 565-606 (1995)).
The application of inorganic nutrient fertilizers is one of the major expenses incurred by producers of high-yielding crop plants (see Good et al., “Can Less Yield More? Is Reducing Nutrient Input Into the Environment Compatible with Maintaining Crop Production?” Trends in Plant Science 9(12):597-605 (2004)). Further, reports have indicated that nitrogen-based fertilizers may be associated with environmental damage (see Vitousek et al., “Human Alternation of the Global Nitrogen Cycle: Causes and Consequences,” Ecol. Appl. 7:737-750 (1997)). Therefore, one important way of decreasing the amount of inorganic nitrogen that is applied to plant crops is to develop ways to improve nitrate use efficiency (“NUE”) in plants.
Traditional plant breeding and marker-assisted selection are techniques that have been investigated for developing and identifying plants with increased NUE (see Good et al., “Can Less Yield More? Is Reducing Nutrient Input Into the Environment Compatible with Maintaining Crop Production?” Trends in Plant Science 9(12):597-605 (2004)). However, these approaches are often time-consuming and labor-intensive. An alternative approach is to use genetic engineering techniques to develop transgenic crop plants that have enhanced NUE. This approach requires the identification of genes that enhance NUE. Efforts have been reported regarding identifying genes that are regulated by nitrogen levels in Arabidopsis (Scheible et al., “Genome-Wide Reprogramming of Primary and Secondary Metabolism, Protein Synthesis, Cellular Growth Processes, and the Regulatory Infrastructure of Arabidopsis in Response to Nitrogen,” Plant Physiol. 136:2483-2499 (2004)). However, there is a need to identify genes that are involved in nitrate uptake and metabolism in economically important crop plants such as corn.
The present invention is directed to overcoming these and other deficiencies in the art.